dmaengine: sh: rz-dmac: Fix lockdep assert warning

Fix the below lockdep assert warning by holding vc.lock for
vchan_get_all_descriptors().

WARNING: virt-dma.h:188 rz_dmac_terminate_all
pc : rz_dmac_terminate_all

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240625170119.173595-1-biju.das.jz@bp.renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>